There are many situations where the indication of very small distances or displacements is needed. For example, precision gauging instruments such as micrometers, dial indicators, feeler gauges, and electronic indicators have been required to provide precise settings or indications of minute displacements on the order of 0.0001 inch (2.5 .mu.m). Replacement of such devices by a displacement indicator system relying only on optical indications of displacement changes discernible by the human eye would have the obvious potential advantages of simplicity in use and lower cost through elimination of peripheral equipment. These advantages would be further enhanced if the replacement system constituted an indicator incorporated within each object for which a precision adjustment is desired to be made.
Early prior art, such as U.S. Pat. Nos. 3,602,186 to Popenoe and 3,799,108 to Mosow, disclose attempts to achieve these advantages by means of indicators which changed color in coordination with a change in length of a fastening device, such as a threaded bolt, undergoing imposition of strain, stress or torque. The color change resulted from changes in light transmittance of a colored indicating fluid surrounding a colored disk at one end of the unstressed reference member due to changes in thickness of that fluid accompanying elongation of the stressed bolt relative to the unstressed member. However, these indicators, although demonstrably improving the art, suffered themselves from high manufacturing cost, complex manufacturing procedures and insufficient precision.
The precision with which an adjustment may be made depends, in turn, on the sensitivity of the indicator used. That sensitivity is related to the optical density of the indicating fluid. In other words, sensitivity relates to the thickness of fluid necessary to absorb some fraction of the light reflected from the colored indicator disk. The higher the optical density of the fluid, the smaller the deflection necessary to effect a change in color from bright to dark. The optical density of the fluid is controlled by the concentration of dye dissolved in the colorless carrier, which is usually a light mineral oil or silicone fluid. There is a limited solubility of the dye in the fluid so that, beyond a certain concentration, the dye will precipitate out at some lower temperature producing undesirable solid particles in the indicating fluid. A base fluid may be prepared by selecting a desired lower temperature limit and, by experimentation, determining the maximum concentration of dye powder in mineral oil which will not show precipitation until that lower temperature limit is reached. For purposes of the preferred embodiment, an arbitrary lower temperature limit of minus 18 degrees C. was selected. It should be understood that other temperature limits may also be used.
In order to define the optimal indicating fluid thickness, it is necessary to determine the relationship between light reflectance and fluid thickness. By measuring photoelectrically the light reflected through a prior art indicator from a red LED (650 nm) light source as a function of indicator thickness (indicating fluid layer thickness) and normalizing the resulting measurements with respect to the light reflected with zero fluid layer thickness, a "reflectance ratio" of the indicator versus the displacement from a zero thickness can be obtained on a semi-log scale. Since the light passing through the absorbing layer of fluid is inverse exponential, it will be linear on a semi-log plot. The slope of this linearized curve describes the optical density of the fluid and thus the sensitivity of the fluid-colored indicator combination. Since the response is similar to a "half-life" to quantify the optical density. Therefore, the fluid layer thickness which will transmit 1/e (about 37%) of the incident radiation is termed t* for purposes of this invention. For each t* thickness of fluid layer, some 63% of the initial radiation will be absorbed within that thickness. The base fluid prepared as above measures at t* of about 100 microinches. Tests on various human subjects show that the brightness or color change produced by a deflection of one t* is the best resolution attainable by the human eye.
Different t*'s can be obtained by adding an appropriate amount of clear dilutant to the base fluid. Thus, by mixing base fluid (t*=100) with an equal volume of diluant mineral oil, a fluid with a t* of 200 will result. Further experimentation has indicated, for example, that by using a bright red indicator disk and a dark blue fluid, microindicators change in color from bright red to black in about 10 t* or 1000 microinches (1 mil). Note that microindicators may display other color changes, such as a change from bright yellow to blue. This change corresponds to a reflectance ratio where less than 1% of the zero-thickness light remains, and most test subjects felt that the last vestige of red coloration had just disappeared so that the indicator could be described as "black" in color.
Prior art design is based on the concept that the microindicator on a loose bolt appears red while that on a tight bolt appears black, with shades in between signifying intermediate tension. According to elastic theory, the bolt strain equals the applied stress over the elastic modulus. The elastic modulus (Young's Modulus) is very nearly constant for a given material, regardless of heat treatment, alloy or bolt size. Thus, the strain or elongation of a bolt in inches per inch, is directly proportional to the applied stress, which is defined as the bolt tension per unit cross-sectional area. This has the effect that all high-carbon steel bolts (Grade 5), which have an ultimate tensile strength of 120,000 psi and when tightened to a stress of 90% of yield strength, or about 75,000 psi, which is typical will elongate about 2.5 thousandths of an inch per inch of bolt length, regardless of bolt diameter. Similarly, alloy steel bolts (Grade 8), which have a higher yield strength, will elongate about 3.4 mils per inch when tightened to the proper tension. Consequently, all bolts of a given grade, when tightened properly, will have nearly identical elongations in the range of 3-4 mils, if measured over the same length, and when actuating pins of lengths typified by this disclosure are used.
In order to make an effective, visual microindicator, a color change from red to black must occur when fluid layer thickness is increased by approximately 3 mils. Since the perceived red-to-black indication has been shown to correspond to about 10 t* displacement, a fluid with a t* of about 300 microinches is required. Additionally, since human color discrimination is limited to distinguishing an indicator color change corresponding to about one t*, an average operator can set a bolt to a tension corresponding to 10 t*, plus or minus about one t*, resulting in a bolt setting accuracy of plus or minus 10 percent. Although this level of accuracy exceeds that obtainable by using torque wrenches or other torqueing methods, it is insufficient for many engineering uses. In such cases, an accuracy as close to 2% as possible is desirable.
Since the perceptivity of the human eye is naturally limited, improved accuracy can only be obtained by improving the sensitivity of the microindicator. By using the most dense base fluid possible at which t*=100 microinches, the average operator can be expected to adjust elongation to within 100 microinches (or 1 t*). Analysis of typical actuating pin lengths and stress-strain relationships indicates that a 100 microinch elongation increment results from a stress increment of 2400 psi. The working stress of high-carbon steel bolt is bout 75,000 psi, while the working stress of an alloy steel bolt is about 100,000 psi. Therefore, using the densest base fluid, an operator can be expected to set tension by eye to within 3.2% (2400/75000) on a high-carbon steel bolt and to about 2.4% (2400/100,000) on an alloy bolt product. The result shows that approximately a four-fold improvement in precision is possible.
In order to obtain this improved precision, the indicator must be biased so that the microindicator is "engaged" only in the final portion of the total elongation, and is "free-wheeling" over the earlier portion. Further goals are ease of manufacture and either self-calibration or extremely easy calibration.
One way of accomplishing these goals is disclosed in U.S. Pat. No. 3,987,699 to Popenoe, FIGS. 7 and 10. In these embodiments, the operation of the indicator as disclosed in earlier patents, such as U.S. Pat. No. 3,602,186, is inverted. Thus, rather than the microindicator thickness increasing with elongation, the components are arranged such that the microindicator thickness decreases with increasing elongation. In this way, when a very dense base fluid (t*=100) is used, the microindicator will appear black when loose, and will continue to be black when tightened until the bolt has elongated such that the indicator thickness is less than 1 mil. Since the average bolt will elongate 3 mils per inch, or about 4 mils over the length of the typical actuating pin when properly tightened, the indicator will remain black until after the bolt is about 75% tightened, and then will gradually turn bright red over the final 25%, yielding the fourfold improvement in accuracy described above.
Several disadvantages may accompany the relevant embodiments of U.S. Pat. No. 3,987,699. If the actuating member or actuating pin is placed internally, as in FIG. 7, the mechanics may become complicated and may be easily damaged unless a double window is used. But, a double window may have the further problem of multiple internal reflections between the windows which would degrade the indication. If the actuating member is placed outside the shank of the bolt, as in FIG. 10, the fastener could become impractical to use due to possible corrosion and damage not present when all elements are embedded within the bolt.